1. Field of the Invention
The present invention relates to a multipoint autofocus system for an optical instrument such as a camera.
2. Description of the Prior Art
A phase difference-type focus detecting system is known to be one of the focusing systems for a camera. In a phase difference-type focus detecting system, an object image within a focus detection zone is split into two images which are received by a CCD line sensor, whereat the images are converted to electrical image signals. The phase difference between the object images is detected in accordance with the electrical image signals which represent the two object images, so that a focus state (defocus) can be determined based on the phase difference thus detected.
In general, the detection of the chase difference and the defocus is carried out by a CPU (Central Processing Unit; Microcomputer). Therefore, the image signals (analogue signals) are converted co digital signals which can be processed in the CPU. The A/D conversion is in general carried out by an A/D converter incorporated in the CPU.
Generally speaking, a sensitivity range of a CCD line sensor is narrower than an object brightness range in which the object can be photographed. Therefore, the light receiving time (integration time) of the CCD line sensor in which light is received is controlled or the amplification rate of the output voltage of the CCD line sensor is adjusted, in accordance with the object brightness, so that an output voltage approximately equal to the saturation output voltage of the CCD line sensor can be obtained regardless of the object brightness.
However, there are various kinds of CCD line sensors having different saturation output voltages. For example, the saturation output voltage of a CCD line sensor is more than one-half the full range of the A/D converter, but the saturation output voltage of another CCD line sensor is less than one-half the full range of the A/D converter. If a CCD line sensor whose saturation output voltage is less than one-half the full range of the A/D converter is used, the range of the converted data is narrow, so that the operation of the CCD line sensor is restricted.
In a multipoint autofocus system in which the focuses of objects in a plurality of focus detection zones can be detected, the object image is received by a line sensor (light receiving means) having a number of light receiving elements (e.g., photodiodes) for each focus detection zone and is converted to electric signals. The accumulated electric charges are integrated. When the integration value of each line sensor is a predetermined value, the integral operation of the line sensor is completed. The integral operation continues until the integration value becomes a predetermined value or a predetermined maximum integration time lapses, whichever is earlier. For a line sensor in which the integration value does not reach a predetermined value after the lapse of the maximum integration time, the predetermined value is replaced by a smaller reference value, and thereafter, the integration value is compared with the new reference value (smaller value) and thus, the gain (degree of amplification) of the integration value is increased. These operations are repeated in steps (stepwisely) until the integration value becomes the reference value which has been reduced stepwise. If the integration value does not eventually reach the reference value, the integral operation of the line sensor is forcedly stopped.
To detect the completion of the integral operation, the integration value of a monitor sensor provided adjacent to each line sensor is used in place of the integration value of the corresponding line sensor. Namely, the integration values of the monitor sensors are monitored to control the integration time of the line sensors, on the assumption that the relationship between the integration values of the monitor sensors and those of the line sensors corresponding thereto is kept constant.
The reference value to terminate the integral operation is set by a reference monitor sensor. The integration value of the monitor sensor could be different from the integration values of other monitor sensors. For instance, if the integration value (absolute value) of a monitor sensor is smaller than the integration value (absolute value) of the reference monitor sensor, the integral operation ends before the integration value of the corresponding line sensor reaches the predetermined reference value, so that the maximum integration value range in which the integration value can be processed cannot be effectively used. Conversely, if the integration value (absolute value) of a monitor sensor is larger than the integration value (absolute value) of the reference monitor sensor, the integral operation continues after the integration value of the corresponding line sensor has reached the predetermined reference value, since the integration value of the monitor sensor does not yet reach the predetermined reference value. Consequently, there is a possibility that the integration value of the line sensor exceeds the maximum integration value range in which the integration value can be processed. Moreover, in the case that the integration value of the reference monitor sensor reaches the predetermined reference value within the maximum integration time but the integration value of other monitor sensor(s) does not reach the predetermined reference value thus resulting in an increase in the gain, the integration value of the corresponding line sensor(s) may exceed the processable maximum integration value range.
In the former case, i.e., if the integration value of a monitor sensor is smaller than the integration value of the reference monitor sensor, since the integration value range is restricted, the integration value on a dark object portion in particular is too small that the image signal contains a large amount of noise (extraneous signal components), thus leading to a reduction in detection precision. In the latter case, i.e., the integration value of a monitor sensor is larger than the integration value of the reference monitor sensor, since the integration value of the line sensor exceeds the processable maximum integration value range, the integration value on a bright object portion in particular fluctuates, thus resulting in a reduction in detection precision.
Recently, various kinds of multi-point autofocusing systems have been introduced in which the focuses of objects within a plurality of focus detection zones are detected using the object distance measuring sensors as mentioned above. For instance, in a known multi-point autofocusing system, the focuses of objects within a horizontally extending central focus detection zone and a pair of vertically extending peripheral focus detection zones on opposite sides of the central focus detection zone, arranged in a generally H-shape on an image surface, are detected.
In this autofocusing system, the focus state of the object within a specific focus detection zone selected by a photographer is detected to perform the focusing operation or the focus state of the objects within the plural focus detection zones is detected and the object distance data determined based on the focus state is compared, so that the focusing operation is carried out to focus on, for example, the farthest or closest object.
However, upon taking a portrait picture or a scene against a blue sky background, if a defocus of the background (blue sky) whose contrast is low is obtained, there is a possibility that the focus control is carried out to focus on the blue sky. Alternatively, upon taking a picture of a person who is standing on the ground (e.g., on a road), if a defocus of the road portion closer to the camera than the person is obtained, the focus control may be carried out to focus on the road portion. Moreover, if no focus can be effectively detected for some focus detection zones, the automatic focusing operation cannot be effected or the focus is adjusted for a wrong object (unwanted or unintended object). Consequently, the intended object to be photographed is out of focus.